1. Field of the Invention
The present invention relates to a toner for use in electrophotography. In addition, the present invention also relates to a developing device, an image forming apparatus, and an image forming method using the toner.
2. Discussion of the Background
Recently, the development of office automation and colorization of documents have been remarkable. The need to print a document including figures (such as graphs made by a personal computer) by a printer and then copying the printed document to prepare materials for use in a presentation, has been increasing, as well as the need to copy conventional full text documents. Because images produced by printers typically include solid images, line images and half tone images, printers are required to produce high quality solid, line and half tone images. In addition, printers are required to have high reliability.
In electrophotography, a developer is at once adhered to an electrostatic latent image formed on an image bearing member (e.g., a photoreceptor), and the resultant image is then transferred from the image bearing member onto a transfer medium such as a transfer paper, and finally fixed on the transfer paper. As the developer configured to develop the electrostatic latent image formed on the image bearing member, a two-component developer including a carrier and a toner and a one-component developer consisting essentially of a toner (e.g., magnetic toner and non-magnetic toner) are known. The two-component developer has the following drawbacks:
(1) Toner particles tend to adhere to the carrier, resulting in deterioration of charging property of the developer; and
(2) Because only the toner is consumed in a developing process, a device for controlling the toner concentration of the developer is needed, resulting in upsizing of a developing device.
In contrast, the one-component developer has no above-mentioned drawbacks while having advantages over the two-component developer such that an image forming apparatus can be small in size, etc. Therefore, the one-component developer has been mainly used recently.
The one-component developers are broadly classified into two categories: a magnetic one-component developer consisting essentially of a magnetic toner and an on-magnetic one-component developer consisting essentially of a non-magnetic toner.
In a magnetic one-component developing method using a magnetic one-component developer, a magnetic toner including a magnetic material (such as ferrite) is held on a developing sleeve, containing a magnetic field generating mechanism (such as magnets) therein, generating a magnetic field by the magnetic force thereof, and then a thin toner layer is formed by a toner layer thickness control member to develop an electrostatic latent image. The magnetic one-component developing method is broadly used recently in compact printers.
On the other hand, in a non-magnetic one-component developing method using a non-magnetic one-component developer, a non-magnetic toner is fed on a developing sleeve by pressing a toner supply roller thereto. Thereby, the toner is held on the developing sleeve due to the electric force, and then a thin toner layer is formed by a toner layer thickness control member to develop an electrostatic latent image. The Non-magnetic one-component developing method is widely used for compact full-color printers because of having the following advantages:
(1) The non-magnetic toner includes no magnetic material which is typically colored, and therefore a toner having good color reproducibility can be provided; and
(2) The developing device can be small in size and the manufacturing cost thereof can be reduced because the developing sleeve includes no magnet.
However, the one-component developing method also has some drawbacks. In the two-component developing method, a carrier stably charges and transports a toner. In other words, a toner is transported to a developing sleeve to be developed after being sufficiently mixed with a carrier in a developing device. Therefore, the toner can be stably charged and transported even after a long repeated use. In addition, the two-component developing method can be easily applied to high-speed machines.
In contrast, in the one-component developing method, there is no mechanism for stably charging and transporting a toner. Therefore, the toner cannot be stably charged and transported especially after a long repeated use, or when used in high-speed machines. In the one-component developing method, a toner is transported to a developing sleeve and then a thin toner layer is formed by a toner layer thickness control member, as mentioned above. In this case, a charging time in which the toner is contact-charged or friction-charged by frictional charging members (such as the developing sleeve and the toner layer thickness control member) is too short. Therefore, weakly or reversely charged toner particles are easily produced in the one-component developing method as compared with the two-component developing method.
In particular, in the non-magnetic one-component developing method, the toner is transported by at least one toner transport device to develop the electrostatic latent image formed on the image bearing member. In this case, the thickness of the toner layer held on the toner transport device is preferably as thin as possible. If the toner layer is too thick, all over the toner layer cannot be uniformly charged, while only surface portion thereof is charged. The non-magnetic one-component developer is required to be quickly charged and maintain an appropriate amount of the charge.
In the non-magnetic one-component developing method, materials used for the toner supply member, the toner layer thickness control member, the developing sleeve, etc., should be selected in view of giving a large amount of charge to the toner, because the toner is charged by pressing the above members thereto. The binder resin, which is the main component of the toner, is required to have good resistance to a mechanical impactive force in order to produce highly reliable images for a long period of the time. The toner tends to generate heat when frictionized, and then adhere to a charging member and shorten the life of the developing sleeve. As a result, the toner cannot be stably charged after long repeated use. Alternatively, the toner particles tend to be pulverized in the developing device, and therefore ultrafine particles are produced. As a result, the resultant image density decreases and white spots appear therein. For these reasons, a process cartridge (i.e., an image forming unit) used for the non-magnetic one-component developing method cannot be used for a long period of the time, and has to be replaced with a new one only after several thousands copies are produced.
In attempting to solve the above problems, published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-A) 02-205858 and 02-198456 have disclosed toners including an initial toner and a supply toner, wherein the kind, amount, surface treatment agent used, etc., of each of the additives is different so that the initial and supply toners have different charge quantity. It is described therein that the toner in the developer can keep a constant charge quantity and such a developer can produce stable images. But the charge quantity of the toner can be controlled only at the first occasion of supplying the supply toner, and cannot be controlled at the second or later occasions. In addition, durability of the toner is insufficient.
The thermal property of the binder resin of the toner is important in the fixing process. A pressure heat fixing method using a heat roller, etc., is widely used for the fixing process. In this method, the surface of the heat roller contacts a toner image formed on a transfer member upon application of pressure thereto to fix the toner image on the transfer member. The toner image can be rapidly fixed on the transfer member because of high thermal efficiency of this method, and therefore this method is preferably used for high-speed machines. When the toner is used for high-speed machines, the toner is required to have low-temperature fixability (a property in which the minimum fixable temperature is low). Such a toner can be obtained using a binder resin having a low softening point.
When the heat roller contacts the toner image, an offset problem in that part of the fused toner image is adhered to the surface of the heat roller, and then the part of the toner image is re-transferred to an undesired portion of the transfer member itself or the following sheet of the paper, etc., tends to occur. In attempting to prevent the occurrence of the offset problem, a method in which a surface of the heat roller is formed by a releasable material, and a method in which a liquid release agent such as a silicone oil is further applied to the surface of the heat roller are proposed. These methods are effective in improving hot offset resistance, but do not contribute to the improvement of low-temperature fixability, separativeness, preservation stability, etc.
In a typical full-color image forming process, at least 3 toner images (i.e., a magenta, a cyan, and a yellow toner images, optionally a black toner image) are independently developed and transferred, and then these toner images are overlaid on a transfer medium. The overlaid toner image is fixed thereon at once to form a full-color image. Since the full-color image consists of plural toner layers and has a large thickness, there are some drawbacks that cracks tends to appear on the image, the glossiness of the image tends to decrease, and image defects tend to occur when the image is folded. For these reasons, the toner for use in the full-color image is required to have an appropriate physical strength and a property of producing high glossiness images.
The fixability of a toner is typically depends on the physical properties of the binder resin, which is the main component of the toner. A styrene-acrylic resin and a polyester resin are typically used as a binder resin of a toner. It is typically known that a toner using a styrene-acrylic resin is suitable for producing low-glossiness images because the styrene-acrylic resin has a high softening point, but the low-temperature fixability and the resultant image strength thereof are less than those of a toner using a polyester resin.
A toner using a polyester resin has good low-temperature fixability, but has poor hot offset resistance. In attempting to improve the hot offset resistance of the toner, methods of increasing surface cohesive force thereof such as increasing the polymerization degree of the binder resin, using a multivalent monomer to be cross-linked, and introducing a gel component to the resin are proposed. However, these methods deteriorate not only low-temperature fixability of the resultant toner but also pulverization property thereof in the manufacturing process. Recently, toners are being modified to have a smaller particle diameter so as to produce high quality images. Since polyester resins and polyol resins have high molecular cohesive force, these resins have poor pulverization property in the toner manufacturing process, and tend to adhere to or aggregate on the inside walls of the pulverizer.
The smaller particle diameter a toner has, the larger van der Waals' force the toner has. Therefore, the toner particles easily aggregate and deteriorate the classification efficiency. As a result, the manufacturing efficiency deteriorates and the manufacturing cost increases.
In attempting to improve pulverization property of a toner, a method in which the molecular weight of a binder resin is decreased is proposed, but thereby the glass transition temperature typically decreases. When the glass transition temperature is too low, the resultant toner has poor preservation stability, i.e., blocking resistance. Further, there are drawbacks that resistance to the offset problem in that a toner adhered to a heat roller is re-transferred to the following sheet of a recording material deteriorates, and the strength of the fixed images deteriorates.
JP-A 02-269364 discloses a polyester resin including diol units of a polyoxyethylene-based bisphenol A and a polyoxypropylene-based bisphenol A. A toner including such a resin has poor low-temperature fixability and preservation stability, and cracks tend to appear on the resultant full-color image.
In attempting to solve these problems, toners including two different polyester resins are proposed. For example, JP-A 60-90344 discloses a toner including a non-linear polyester resin and a linear polyester resin. JP-A 64-15755 disclose a toner binder resin including a cross-linked polyester resin having a glass transition temperature of not less than 50° C. and a softening point of not greater than 200° C. and a straight-chain polyester resin having a softening point of not greater than 150° C. and a weight average molecular weight of from 3,000 to 50,000. JP-A 02-82267 discloses a toner including anon-linear high-molecular-weight polyester resin having a weight average molecular weight (Mw) of not less than 5,000 and a dispersion degree (Mw/Mn) of not less than 20 and a non-linear low-molecular-weight polyester resin having a weight average molecular weight (Mw) of from 1,000 to 5,000 and a dispersion degree (Mw/Mn) of not greater than 4. JP-A 03-229264 discloses a toner including a linear polyester resin having an acid value of from 5 to 60, a non-linear polyester resin having an acid value of less than 5, and an organic metal compound. JP-A 03-41470 discloses a toner including two different saturated polyester resins in which the ratio of the acid value there between is not less than 1.5. Although each of these toners has good fixability, impactive force resistance thereof is insufficient for use in the one-component developing method.
In attempting to solve this problem, JP-A 2004-163836 discloses a toner including a polyester resin having a specific diol units and another polyester resin having a different molecular weight. However, fixability and preservation stability thereof are insufficient.
As mentioned above, the non-magnetic one-component developing method has an advantage that the developing device can be small in size. But a toner used for the non-magnetic one-component developing method, which has a good combination of durability, fixability, preservation stability, and pulverization property, is not provided yet.